Self-oscillating mixer using tunable long lines



Nov. 5, 1957 I A. BOEKHORST SELF-OSCILLATING MIXER USING TUNABLE LONGLINES Filed Oct. 25. 1954 INVENTOR ANTONIUS BOEKHORST AGENT UnitedStates Patent SELF-OSCILLATING MIXER USING TUNABLE LONG LINES AntoniusBoeithorst, Eindhoven, Netherlands, assignor, by

mesne assignments, to North American Philips Company, inc, New York, N.Y., a corporation of Delaware Application October 25, 1954, Serial No.464,580

Claims priority, application Netherlands December 9, 1953 3 Claims. (Cl.250-20) The invention relates to circuit-arrangements for changing thefrequency of oscillations of very high frequencies, for example, of theorder of c./s., such as are used for television, comprising aself-oscillating mixing tube, more particularly a mixing triode, betweenthe control-grid and cathode of which a circuit tuned to the oscillatorfrequency is connected. In order to counteract as such as possible theradiation of oscillations of oscillator frequency from the antenna insuch circuit-arrangements, the incoming oscillations, in a known circuitarrangement, are supplied to an inductance connected between a point ofthe coil of the oscillatory circuit, which point does not carry anyappreciable voltages of oscillator frequency relative to earth, and thecathode of the mixing tube.

It is customary to derive the intermediate frequency oscillations from acircuit, tuned to the said frequency, in the anode circuit of a mixingtube and to make provision of feedback between said circuit and the gridcircuit, thus maintaining the oscillations of oscillator frequency. Ingeneral, the intermediate frequency is of the order of 10 c./s..

The present invention has for its object a circuit of this type which isso arranged as to be tunable in a simple manner over a comparativelywide frequency range, whilst the emission of oscillations of oscillatorfrequency from the antenna circuit remains low for any tuning evenwithout the use of a preceding high-frequency amplification stage. Afurther object is to provide such an arrangement that the differencefrequency between the incoming frequency and the oscillator frequencyremains substantially constant throughout the range of tuning.

The invention consists in that not only the inductance of the oscillatorcircuit but also that of the receiving circuit are tunable Lecher wires.

Preferably, the inductance of the oscillator circuit is a Lecher wiresystem comprising two substantially parallel wires, whilst theinductance of the signal circuit is a single wire arranged symmetricallywith respect to the first-mentioned Wires, a single conductive slidingmember being provided by which the three wires are interconnected atcorresponding points.

Furthermore, in accordance with the invention, variable impedances, moreparticularly inductances connected in series with the Lecher wires,permit satisfactory adjustrnent throughout a wide frequency range.

In order that the invention may be readily carried into effect, it willnow be described, by way of example, with reference to the accompanyingdrawing in which Fig. 1 shows a known circuit arrangement,

Fig. 2 shows a circuit arrangement according to the invention, and

Fig. 3 is a variation of the arrangement shown in Fig. 2.

In'Fig. l, V stands for a triode mixing tube. A coil L1 is included inthe antenna circuit and coupled with a coil L2 which may be ahigh-frequency tuning coil. La stands for the tuning coil of theoscillator, which is inductively coupled to coil L4 which, in serieswith a fixed capacitor C4 and an inductance L5, is connected 2,812,432Patented Nov. 5, 1957 between the anode and earth. This coil serves forregenerative feedback, thus producing oscillations in the tube. Thecathode of the tube is earthed. The inductance L5 has for its purpose tomake the anode circuit inductive with regard to incoming high-frequencyoscillations so as to reduce, in combination with the capacitancebetween anode and grid, the damping of the high-frequency tuningcircuit. L5 is the coil of the first intermediate frequency tuningcircuit, which is coupled to L7. Ls is tuned to the intermediatefrequency by means of a capacitor C4 and the available parasiticcapacities. The coil L7, which may also be tuned to the intermedaitefrequency, is connected to the intermediate frequency amplifier.

The upper end of coil L3 is connected to the control grid of tube V, itslower end being connected to the earthed cathode through a capacitor C2.

The upper end of coil L2 is connected to a tapping point A of coil L3,its lower end being earthed through the capacitor C1. The lower end ofcoil L2 is moreover connected, through a capacitor C5, to the commonpoint of a coil L6 and a resistor R2. The static direct current issupplied to the anode of tube V through the said resistor R2. Across thelatter and the capacitor C1 an intermediate frequency voltage is set upand applied via L2 and L3 to the control grid of the triode so as toreduce the damping of the intermediate frequency circuit, whilst a lowfrequency inverse feedback, suppressing over-excitation, is produced bythe same way. The capacitor C2 is a trimmer by means of which the lowestpossible oscillator emission is adjusted at point A. The capacitor C2 isthe oscillator-circuit capacitor. The capacitors C4 and C5 act at thesame time as blocking capacitors with regard to direct voltage. Theresistor R1 serves to obtain the correct bias of the control grid oftube V.

In accordance with the invention, the arrangement is tunable over afrequency range of a given width, for example, the television band offrom 174 to 216 m. c./s., since the inductances L2 and L3 are variableand constituted by Lecher wires.

A circuit-arrangement as described is shown in Fig. 2, correspondingparts bearing the same reference numerals as in Fig. l.

In the arrangement shown in Fig. 2, the inductance L3 is constituted bytwo parallel conductors along which a short-circuiting bridge B ismovable. The conductor L2 provides the self-induction of thehigh-frequency receiving circuit and is arranged symmetrically withregard to the first-mentioned conductors. As shown in this figure, thefirst-mentioned conductors each provide one half of the self-inductionrequired for the oscillator circuit. The conductor L2 is alsoconductively connected to the shortcircuiting bridge B and to this endthe latter is T-shaped. If the short-circuiting bridge is displaced,both the inductance L3 and the inductance L2 will be changed in value,Whilst the upper end of the high-frequency coil L2 remains invariablyconnected to the centre of the oscillator coil L3.

By connecting one half of coil L3 to an additional variable inductanceL8 an oscillator voltage equal to zero is obtainable for two frequenciesof the tuning range at point A. At the lowest value of L3 adjustment iseffected by means of L3 and at the highest value by means of C2, so thatthe total deviation from correct adjustment is very small throughout thetuning range.

In order to reduce the padding deviation recourse may be had to asimilar step in respect of the high frequency coil L2. By means of anadditional inductance L9 the tuning frequency is adjustable on thehigher frequency side, whereas adjustment on the lower frequency side ofthe range is effected by means of the capacitor C1.

As shown in Fig. 2, the coils Li and L4 are loops located substantiallysymmetrically with regard to the circuits with which they areassociated. l

Another solution of the problem involved in obtaining a constantfrequency difference between the receiving circuit'and the oscillatorcircuit is to curve the conductors into parts of circles so that L2. hasa smaller radius than the two halves of L3. An arrangement of this typeis shown in Fig. 3, the spacing between L2 and L3, and the radius of thecircle segments determining satisfactory padding.

What is claimed is:

l. A tunable self-oscillating mixer, circuit comprising a triode tubecontaining cathode, grid and anode electrodes, first and secondconductors positioned substantially sideby-side and mutually paralleland respectively connected electrically at corresponding ends thereof tosaid grid and said cathode, a third conductor positioned symmetricallyand parallel to said first and second conductors and connectedelectrically to said cathode at the end thereof corresponding to saidends of the first and second conductors, a single electricallyconductive slider member positioned to contact said three conductors andadapted to slide along the length of said conductors, a feedback circuitconnected 7 4 to said anode and coupled electrically to at least one ofsaid first and second conductors to cause a local oscillation to occurat the frequency to which said first and second conductors are tuned bymeans of said slider member, and a source of input oscillations coupledto said third conductor.

2.v A circuit as claimed in claim 1, including a padding inductanceinterposed in series with at least one of said conductors at said endthereof.

3. A circuit as claimed in claim 1, in which said conductors are curvedto form circular segments about a common point, the radii of said firstand second conductors about said common point being equal to each otherand larger than the radius of said third conductor about said commonpoint.

References Cited. in the file of this patent UNITED STATES PATENTS2,126,541 De Forest Aug. 9, 1938 2,665,339 Leng Jan. 5, 1954 FOREIGNPATENTS 698,397 Great Britain Oct. 14. 1953

